1. Field of the Invention
The invention relates to microphone circuits, and more particularly to eliminating a popping noise for microphone circuits.
2. Description of the Related Art
A microphone transducer, such as an electret condenser microphone (ECM), converts a sound to a voltage signal. A microphone transducer, however, has weak driving ability and cannot effectively pass the voltage signal to a subsequent circuit with a higher impedance. The microphone transducer therefore requires a pre-amplifier circuit, which has a greater driving ability to pass the voltage signal generated by the microphone transducer to the subsequent circuit.
Referring to FIG. 1, a block diagram of a conventional microphone circuit 100 is shown. The microphone circuit 100 comprises a transducer 102, a biasing resistor 104, and a pre-amplifier 106. After the transducer 102 generates a voltage signal at a node 120, the preamplifier 106 amplifies the voltage signal with a unit gain to generate an output signal Vo at a node 122. The transducer 102 and the biasing resistor 104 are coupled between the node 120 and a ground VGND. The biasing resistor 104 maintains a stable offset voltage at the node 120. In one embodiment, the biasing resistor 104 has a resistance ranging between 100 MΩ and 100 GΩ.
The pre-amplifier 106 requires external power supply for amplification of the output voltage. When the microphone circuit 100 is reset, a biasing voltage is applied to the pre-amplifier 106, temporarily increasing the voltage at the node 120 and resulting in a popping noise. Referring to FIG. 2, a circuit diagram of a conventional microphone circuit 200 is shown. The pre-amplifier 106 of the microphone circuit 100 is modeled as a pre-amplifier 206 comprising a loading resistor 244, an N-type JFET transistor 242, and a capacitor 246. The transducer 102 of the microphone circuit 100 is modeled as a transducer 202 comprising a signal source 232 and a capacitor 234. The biasing resistor 204 is equivalent to the biasing resistor 104.
The capacitor 246 indicates a parasitic capacitance between a gate and a drain of the JFET transistor 242 and ranges between 200 fF and 1 pF. The capacitor 234 of the transducer 202 has a capacitance ranging between 5 pF and 10 pF. When the microphone circuit 200 is reset, a biasing voltage VDD of 2V is applied to a terminal of the loading resistor 244, resulting in a voltage of 1.67V at the node 222 and inducing temporary voltage increase ΔV of about 64 mV at the node 220 according to following algorithm:ΔV=1.67V×[C246/(C246+C234)]=1.67V×[200 fF/(200 fF+5 pF)]=0.64 mV,
wherein C246 is capacitance of the capacitor 246, and C234 is capacitance of the capacitor 234.
Referring to FIG. 3, a schematic diagram of the voltage at the node 220 of FIG. 2 during resetting is shown. When the biasing voltage VDD of 2V is applied to the loading resistor 244 of the pre-amplifier 244 at time T0, the voltage at the node 220 is raised to 64 mV at time T1 and then gradually reduced to a converge voltage of 0V. A converge time TC of 400 ms is calculated according the following algorithm:TC=R204×(C246+C234)×8=400 ms,
wherein R204 is resistance of the biasing resistor 204, C246 is capacitance of the capacitor 246, and C234 is capacitance of the capacitor 234.
A typical ECM microphone with a diameter 4 mm has a sensitivity of −44 dB Vrms/Pa, wherein Pa is a unit of air pressure and 1 Pa is equal to a 94 dB sound pressure level. The temporary voltage increase ΔV of 64 mV at the node 220 therefore generates a popping noise equal to a 105 dB sound pressure level. In comparison with conversation of a 60 dB sound pressure level and rock-and-roll music of a 94 dB sound pressure level, the popping noise induced by resetting the microphone circuit 200 has a much greater sound pressure level of 105 dB and requires a long converge period of 400 ms before being settled. The popping noise therefore grades performance of the microphone circuit 200. Thus, a method for preventing a microphone circuit from generating a popping noise when being reset is therefore required.